1. Technical Field
This invention relates to ignition plugs, such as igniter and spark plugs, used in internal combustion engines to ignite the combustion gases in the combustion chamber, and more particularly to the seal provided between the insulator and the electrode which serves to prevent the escape of combustion gases through the plug.
2. Related Prior Art
A hermetic seal is required between the internal electrode and surrounding tubular insulator body of igniter and spark plug devices for preventing high temperature, high pressure combustion gases from passing through the plugs. Some seals are typically made by fusing glass to the adjacent surfaces of the electrode and insulator. The glass fusion process is labor intensive, costly and subjects the components of the plug to elevated temperatures above 1000.degree. F.
The internal electrodes of such plug devices are typically of a multi-part construction. The electrode components, including upper and lower electrodes, are assembled within the insulator in abutting contact across axially opposed electrode surfaces of the components. While applying an axial compressive load to the components, the confronting surfaces are locally heated and melted by resistance welding to fuse the material of the electrode components together across the interface. Resistance welding imparts only localized heating of the electrode components at the weld interface, and avoids subjecting the other components to high temperatures associated with the glass fusion sealing. With resistance welding, the mating electrode components are locally melted and the materials fused together at the interface without the use of any foreign filler metal. Joining techniques such as brazing and soldering employ low melting point filler metals different than that of the electrode materials being joined, which could impair the electrical properties of the electrode.
U.S. Pat. No. 2,874,208 to Pierce discloses an igniter plug having a single piece center electrode formed with an enlarged firing button at its lower end that is external to the surrounding tubular insulator body. A sleeve of heat-expandable metal is disposed about the center electrode and is urged against an interior shoulder of the insulator by a tensioning nut that is either threaded or brazed about the center electrode. A washer is compressed between the firing button and the external end face of the insulator body to provide a gas-tight seal. There is no teaching or suggestion of incorporating such a compressed washer sealing system in ignition plugs having multi-piece electrodes joined by resistance welding.